The reactive selenium donor compound required for the specific insertion of selenocysteine into proteins and for synthesis of 2-selenouridine in tRNAs has been identified as monoselenophosphate. The authentic chemically synthesized compound prepared by R. Glass and the SELD enzyme product were indistinguishable by 31P NMR and ion pairing HPLC. Both compounds serve as selenium donor for the enzymic conversion of 2- thiouridine residues in TRNAS to 2-selenouridine. Lysine, glutamate, and glutamine TRNAS labeled with 77Se in the 5-methylaminomethyl-2- selenouridine residues of their anticodons were purified from Clostridium sticklandii. Individual species are needed for [77Se] NMR studies on codon-anticodon recognition. The reductive deamination of glycine by clostridia involves the intermediate formation of Se-carboxymethylselenoprotein A. Reductive cleavage of this selenoether by protein C results in the conversion of protein C to an S-acetylthiol ester. To study this novel reaction [14C]CM-Se-protein A was prepared from pure Se-protein A and Br[14C]acetate and used as substrate. Catalytic activities of Protein C from two different clostridia varied markedly. Subunit composition of protein C was determined by SDS PAGE analysis to detect sensitivity to various chemical treatments. In contrast to glycine reductase and several other selenoenzymes that contain selenocysteine residues in peptide linkage, nicotinic acid hydroxylase contains selenium in the form of a dissociable cofactor. Inactivation of the enzyme by alkylation causes dissociation of the selenium as a dialkylselenide. Current studies by Gladyshev and Khangulov show that an EPR signal characteristic of a Se ligand to the Mo of the molybdopterin cofactor is present in the enzyme.